The process for recovery of elemental metal values, such as copper or nickel, from ores and processing liquids by solvent extraction-electrowinning (hereinafter "SX-EW") is well known. See, for example, U.S. Pat. No. 4,484,990 (Bultman et al.). Metal-bearing aqueous solution is obtained by dissolving from an ore the desired metal in an aqueous leach liquor. The resulting solution of metal values is mixed with a water-immiscible organic solvent (e.g. kerosene) containing a water-insoluble ion exchange composition having selective affinity for the desired metal values. The aqueous and organic phases are separated. The desired metal values are removed from the organic phase (which contains the ion exchange composition and the extracted metal values) by mixing with an aqueous strip solution containing strong acid such as sulfuric, phosphoric, or perchloric acid, and having lower pH than the metal-bearing aqueous solution. The aqueous strip solution extracts the desired metal values into the aqueous phase. After separation of the organic and aqueous phases, the desired metal values are present in the aqueous strip solution, and the resulting metal-enriched strip solution is usually referred to as "electrolyte" or "pregnant electrolyte". The desired metal is recovered in purified form by electroplating the metal from the electrolyte. After recovery of the desired metal, the metal-depleted electrolyte is usually referred to as "spent electrolyte". Such spent electrolyte can be recycled as aqueous strip solution for fresh loading with metal values by mixing with loaded organic.
During the electrowinning step, elemental metal is plated out at the electrowinning cathode and oxygen evolves at an insoluble anode. The evolution of oxygen gas forms bubbles which entrain strong acid electrolyte, carrying it into the air above the electrowinning tank in the form of a fine mist or spray when the bubbles break. This mist or spray then spreads throughout the electrowinning tankhouse. The acidic mist is corrosive and a health hazard and can cause extreme discomfort to the skin, eyes, and respiratory systems of tankhouse workers, especially during hot weather conditions.
Certain fluorochemical surfactants have been used in chromium plating baths to promote the formation of a foam at the surface of the plating bath. This foam is said to effectively eliminate the formation of chromic acid mist. Such fluorochemical surfactants are described, for example, in the Brown et al. U.S. Pat. Nos. 2,750,334, 2,750,335, 2,750,336, and 2,750,337. Such surfactants proved unsatisfactory for inhibiting acidic mist formation above electrowinning tanks used in the SX-EW process. For example, the conventional chromium plating fluorochemical mist suppressant C.sub.8 F.sub.17 SO.sub.3 K gave good initial foam formation and mist suppression above a copper electrowinning tank, but the fluorochemical was rapidly extracted into the organic phase during recycling of the electrolyte. In addition, the fluorochemical surfactant C.sub.8 F.sub.17 SO.sub.3 K was found to interfere with copper recovery and to retard phase separation between organic and aqueous phases when used with ion exchange compounds such as "Acorga P5300" (commercially available from Imperial Chemical Industries, Ltd.) and "LIX 64N" (commercially available from Henkel Corporation).
U.S. Pat. No. 2,913,377 (Brown et al.) describes the use of certain perfluoroalkane sulfonic acids to minimize the formation of mist and spray during anodization.
U.S. Pat. No. 4,484,990, supra, discloses a process for recovery of metal values by the SX-EW process wherein certain fluoroaliphatic surfactants are used in the electrolyte to provide mist-inhibiting foam on the surface of the electrolyte. See also, Pike and Johannessen, "Use of Fluorosurfactants for Mist Control in Copper SX-EW," prepared for presentation at a symposium on hydrometallurgy in Santiago, Chile, November, 1985, and Johannessen, Maes, Pike, and Seward, "Aspects of Fluorosurfactant Use for the Control of Acid Mist in SX-EW Operations," prepared for presentation at the Annual Meeting of the Arizona Conference of AIME in Tucson, Ariz., December, 1985.